Web Worker tests fail in Firefox 4/5

[ljharb]

The Web Worker tests fail in Firefox 4/5:

the xpcwrappednative_nohelper "[Exception... "Not enough arguments [nsIDOMNSEventTarget.addEventListener]"  nsresult: "0x80570001 (NS_ERROR_XPC_NOT_ENOUGH_ARGS)"  location: "JS frame :: http://localhost:8000/test/worker-test.js :: <TOP_LEVEL> :: line 32"  data: no]" was thrown, throw an Error :)
([object Object],[object XPCWrappedNative_NoHelper])@http://localhost:8000/node_modules/mocha/mocha.js:4766
([object XPCWrappedNative_NoHelper])@http://localhost:8000/node_modules/mocha/mocha.js:5036
done([object XPCWrappedNative_NoHelper])@http://localhost:8000/node_modules/mocha/mocha.js:4493
((function (err) {test = self.test;if (err) {if (err instanceof Pending) {self.emit("pending", test);} else {self.fail(test, err);}self.emit("test end", test);if (err instanceof Pending) {return next();}return self.hookUp("afterEach", next);}test.state = "passed";self.emit("pass", test);self.emit("test end", test);self.hookUp("afterEach", next);}))@http://localhost:8000/node_modules/mocha/mocha.js:4516
((function (err) {test = self.test;if (err) {if (err instanceof Pending) {self.emit("pending", test);} else {self.fail(test, err);}self.emit("test end", test);if (err instanceof Pending) {return next();}return self.hookUp("afterEach", next);}test.state = "passed";self.emit("pass", test);self.emit("test end", test);self.hookUp("afterEach", next);}))@http://localhost:8000/node_modules/mocha/mocha.js:4946
()@http://localhost:8000/node_modules/mocha/mocha.js:5029
next((void 0))@http://localhost:8000/node_modules/mocha/mocha.js:4871
()@http://localhost:8000/node_modules/mocha/mocha.js:4881
next(0)@http://localhost:8000/node_modules/mocha/mocha.js:4816
()@http://localhost:8000/node_modules/mocha/mocha.js:4848
timeslice(81)@http://localhost:8000/node_modules/mocha/mocha.js:6391

[ljharb]

They also seem to fail in Firefox 14/18/20, but with an error about Map#keys not being a function.